1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus that use a two-component developer and employ electrophotography.
2. Description of the Related Art
An image forming process using an electrostatic photography commonly includes a charging step, an exposure step, a developing step, a transfer step, a cleaning step, a charge removing step, and a fixing step. At a step of forming an image, for example, a surface of a photoreceptor drum that is rotationally driven by a charging device is uniformly charged, and a laser beam is irradiated to the surface of the photoreceptor drum charged by an exposure device, then an electrostatic latent image is formed. Subsequently, the electrostatic latent image on the photoreceptor drum is developed by a developing device, and a toner image is formed on the surface of the photoreceptor. The toner image on the photoreceptor drum is transferred onto a transfer material by a transfer device, then heated by a fixing device, and the toner image is thereby fixed onto the transfer material. Additionally, a residual toner remained on the surface of the photoreceptor after transfer is removed by a cleaning device and collected by a predetermined collecting section, while a residual charge is removed with a charge removing device from the surface of the photoreceptor after cleaning for preparing for next image formation.
As a developer that develops the electrostatic latent image on the photoreceptor, a one-component developer composed of only a toner or a two-component developer composed of a toner and a carrier is commonly used. The one-component developer does not use the carrier, thus does not need a stirring mechanism or the like for mixing the toner and the carrier uniformly, and has an advantage such that the developing device comes to be simply configured, however, has a disadvantage such that a charging amount of a toner is hard to be stabilized, or the like. The two-component developer needs the stirring mechanism or the like for mixing the toner and the carrier uniformly, thus has a disadvantage such that a configuration of the developing device becomes complicated, however, is often used for a high-speed image forming apparatus and a color image forming apparatus because of having excellent stability of the charging amount.
As a developing device using the two-component developer, a developing device including a developer tank, a developing roller, a stirring member, a regulating member, a chute and the like is commonly used. The developer tank supports the developing roller and the stirring member so as to be rotatable as well as reserves the developer inside thereof. The developing roller bears a developer layer on the surface thereof and rotates, and supplies the toner to the electrostatic latent image on the surface of the photoreceptor to form the toner image. The stirring member stirs the developer in the developer tank uniformly to convey toward the developing roller. The regulating member regulates a layer thickness of the developer layer on a surface of the developing roller.
Further, as a developer that develops the electrostatic latent image on the photoreceptor, a one-component developer composed of the toner as a main component, or a two-component developer composed of the toner and the carrier as main components is commonly used. The one-component developer does not use the carrier, thus does not need the stirring mechanism or the like for mixing the toner and the carrier uniformly, and has an advantage such that the developing device comes to be simply configured, while having a disadvantage that the charging amount of the toner is hard to be stabilized, or the like. Therefore, the two-component developer is often commonly used for the high-speed image forming apparatus and the color image forming apparatus.
On the other hand, in the two-component developer, when a toner content rate (hereinafter, referred to as toner concentration) changes in the developer on the developing roller, image concentration of an image developed at a part where toner concentration changes, and the developing device employing the two-component developer is thus required to develop on the photoreceptor with the developer whose toner concentration is always uniform. Consequently, the developing device stirs the two-component developer (carrier and toner) sufficiently to uniform the toner concentration, and thereafter supplies the toner with the developing roller to the surface of the photoreceptor.
However, there is a problem of image defection, what is called, development hysteresis that, when the two-component developer in which the toner is consumed because of being provided for development (that is, including a part where the toner concentration changes) is provided for development again from the developing roller to the surface of the photoreceptor without separating from a surface of a developing roller sleeve, the image concentration becomes ununiform at the part where the toner concentration changes.
Such a problem is caused by a developer that has been remained on a sleeve of the developing roller after development or a toner with low charging in many cases, and there is a need to improve peeling properties of the developer on a developing sleeve or to remove the remained toner of the developing sleeve.
For example, in Japanese Unexamined Patent Publication JP-A 2000-214668, a magnet sheet is provided on an upstream side of the flow of the developer on a developer return guiding plate, and the developer is handed over the developer return guiding plate until nothing is left due to an effect of a line of magnetic force generated by the magnet sheet and a first magnet.
In the developing device described in the JP-A 2000-214668, the developer is easy to be magnetically attracted to the magnet sheet, and particularly in a device that is used by varying speed of the developing roller, it is hard to set magnetic force of the magnet sheet, and a magnetic force vector is different between an end part and a flat part of the magnet sheet, thereby posing a problem such that it is easy to progress retaining of the developer from a leaking magnetic field part to the direction of a plate thickness thereof.